1. Field of the Invention
The present invention is directed to a woven wire screen cloth having rectangular openings for filtering solid particles. In particular, the present invention is directed to a woven wire screen cloth having rectangular openings which will maximize conductance and maximize screen life.
2. Prior Art
In the drilling of subterranean wells, it is often times standard practice to insert a fluid such as an oil well drilling fluid or "drilling mud" which is used to reduce friction in the drilling and which also is used as a carrier fluid. A drilling fluid, which may be aqueous-based or oil based, is inserted into the well during the drilling operation. This may be done through a drilling string of tubing or in another manner. The drilling mud is introduced to the lower most section of the tubing near the drill bit. The drilling mud circulates upward and carries with it cuttings and other particulate material resulting from penetration of the bit through the earth.
It is preferable and desirable to reuse or recycle this drilling mud. This is done by separating the solid particulate matter in the drilling mud from the base fluid. This is accomplished at the surface by using one or more solid separating devices such as vibrating screen machines or "shale shakers" to separate solids from fluids. The vibrating screen machine vibrates a screen or screen panels in a continuous or in varying patterns such that solids larger than the screen mesh openings will not pass through the openings. The solid particulate matter above a "cut point" is thereby separated from the drilling fluid. The continuous vibration, however, combined with the solids striking the screen, tends to reduce the screen usable life.
Various types of screens or perforated plates have been employed in the past although screens composed of woven wire have proven both effective and cost efficient. In woven wire screens, warp wires run lengthwise during the weaving process and are crossed at right angles by the shute wires. The woven wires intersect and form openings in the cloth. The mesh count is the number of openings per linear inch of screen. It is known that making the openings rectangular or oblong will increase the conductance. The average length to width ratio of the screen openings is called the aspect ratio.
Increasing the length of the openings beyond a certain point will result in decreased efficiency since the wires will tend to move in directions perpendicular to the length. Various efforts have been addressed to this problem. For example, see Hermann, U.S. Pat. No. 2,052,467, wherein the shute wires are eliminated and the warp wires tensioned in effort to avoid movement.
Another proposal directed to this problem is seen in Cagle, U.S. Pat. No. 5,256,291, wherein the shute wires are double woven.
Other proposals directed to this problem include coating the woven cloth with a bonding agent, which tends to lock the intersections in place.
The screen panels in the vibratory screen machines are often times replaceable and may be of a single woven wire screen layer or of multiple screen layers. In a frequently used procedure, a three layer screen assembly is utilized, consisting of two fine mesh layers and a coarser supporting layer. Further support in the nature of a perforated metal plate may also be employed.
The multiple layer screen assembly serves a number of purposes. The multiple layers serve as support for the finest screen layer. Additionally, the multiple layers address the possibility of blinding, which is the tendency of solid material being screened to clog in the openings of the screen cloth or screen cloths. During the vibratory process, the two fine screen layers will strike against each other, tending to unclog the clogged openings.
The separation performance of a screen assembly is represented by its separation performance, its conductance or liquid through-put performance and its service life or durability. The separation performance of a screen assembly, the percent of solids removed as a function of particle size, is often measured. The liquid through-put capacity is primarily a function of screen conductance and its usable area. Conductance is a measure of the ease with which fluid can flow throughout the screen per unit area. Conductance is calculated from the mesh count of the wires and the wire diameters of the screen cloth according to a known formula (see API Recommended Practice 13E, Third Edition, 1993). Conductance, C, measured in kilodarcies/millimeter is computed by the formula ##EQU1## Where "E" is the void fraction of the screen, "A" is the wire surface area to mesh volume ratio and "t" is the screen thickness. "E" is given by a formula ##EQU2## Where N.sub.s and N.sub.w are shute and warp mesh counts and V.sub.s and V.sub.w are the wire volumes in cubic inches.
The wire surface area to volume ration, A, is given by the formula ##EQU3## Based on these formulas, it is possible to calculate the conductance through the screen.
For multi-layer screens, the inverse of conductance for each screen layer is summed to equal the inverse of the net overall conductance as follows: ##EQU4##
It is known that increasing the wire diameter size of the filaments will increase the service life of the screen. At the same time, increase of the wire diameters will decrease the conductance through the screen.
It will be recognized that service life will also be influenced by other factors such as the rate of loading on the screens and the abrasiveness of the cuttings.
Also, in the past, it has been known to calendar screens by placing the screen layer through opposed rollers although, heretofore, this has been done on larger diameter screens to flatten out the knuckles at the intersections.
It is, therefore, a goal of the present invention to balance enhanced screen life while maximizing the conductance of the screen at a reasonable cost of manufacture.
Accordingly, it is a principal object and purpose of the present invention to provide a screen with rectangular mesh openings that will maximize conductance or liquid through-put as well as enhance the screen life of the screen assembly.
It is a further object and purpose of the present invention to optimize the wire diameters while maximizing conductance or liquid through-put as well as enhance the screen life of the screen assembly.
It is a further object and purpose of the present invention to calender the screen cloth to assist in locking the intersections of the warp and shute filaments in place.